A copyright is a document that gives the ownership rights over artistic work, like songs or stories. If you don't get a copyright for your brilliant new novel, someone could adapt it into a movie without paying you a dime.
Copyright means exactly what it sounds like: the right to make copies of something. If you copyright a song, a singer will to have to pay you to record it. Usually, copyright only lasts for a certain number of years. In the U.S. when a book or song has had a copyright for more than 75 years, that expires and it becomes "public domain," which means nobody owns the rights to it anymore.